This invention relates to gas turbines, and more particularly, to a liquid fuel nozzle assembly having a passive protective purge for protecting the nozzle from damage when neither water nor active purge is flowing through the water passageway of the nozzle.
Industrial gas turbines often are capable of alternatively running on liquid and gaseous fuels. These gas turbines have fuel supply systems for both liquid and gas fuels, e.g., natural gas. The gas turbines generally do not simultaneously burn both gas and liquid fuels. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off and vice-versa. The fuel/water system that is not being used is usually purged with air.
Gas turbines that burn liquid fuels require a liquid fuel purge system to clear combustor fuel nozzles of liquid fuel. Air is typically used to actively purge unused combustor nozzle passages during turbine operation. Purge air supplied from an external source that is regulated by a control valve. When the fuel/water system is activated or deactivated during turbine operation, the purge air needs to be turned on or off. The purge air and fuel or water are generally not permitted to be “on” at the same time for safety reasons. Thus, when activating a fuel/water system, the purge air is shut “off” before the fuel is turned on, and when deactivating a fuel system, the fuel is shut “off” before the purge air is turned “on”. This sequence results in a brief period during which there is neither fuel/water flow nor purge air, thus opening the possibility of ingesting combustion gases into the fuel nozzle assembly. The brief no-flow period may also cause thermal distress of the fuel nozzle.
In one existing approach, the time-period during which both the fuel/water and-purge air are “off” is minimized for preventing fuel nozzle damage during a brief no-flow period. There are practical limits, however, as to how short this time-period can be set. Also, flowing purge air quickly into a fuel system that is full of fuel due to a prior operation may force the left over fuel into the combustor and produce an undesirable surge in turbine output. Further, the turbine control system requires some minimum time to confirm that a fuel valve or a purge air valve is in fact closed before opening the control valve to the other. The flowing of purge air quickly into a water system that is full of water due to prior operation, may force the left over water into the combustions system in this case producing the undesirable flaming out of the combustion system.
In another existing approach, a portion of combustor inlet air is diverted to wash over an outer exit of a fuel nozzle passage so that any back-flow into the nozzle will consist only of air and not hot combustion products. This approach, however, is not only difficult to implement but also includes performance side effects.
The systems that control the air purges and liquid flows are both complex and expensive. Currently, there are separate purge supplies to the atomizing air, liquid fuel (e.g., oil), and water passages. Currently, the oil and water passages are actively purged whenever oil or water are not flowing. The oil purge has shown to be crucial to thermally protect the oil tip and also to scavenge oil out of the oil passages following oil operation. While the water tip also requires thermal protection, there is no need to scavenge residual water from the water passages, as it cannot cause a problem equivalent to oil coking. With no water scavenge requirement, an independent or dedicated externally applied purge of the water passage is not necessary provided the water tip can be thermally protected by other means.
Therefore, there is a need for a system and method for a water nozzle that will thermally protect the water nozzle tip and eliminate a need for a dedicated externally supplied air purge of the water passages.